AU643373B2

AU643373B2 - Polybutene-1 resin laminate

Info

Publication number: AU643373B2
Application number: AU12868/92A
Authority: AU
Inventors: Akito Nishimura; Haruhiko Tanaka
Original assignee: Mitsui Petrochemical Industries Ltd; Shell Oil Co
Current assignee: Mitsui Chemicals Inc; Shell USA Inc
Priority date: 1991-03-20
Filing date: 1992-03-12
Publication date: 1993-11-11
Anticipated expiration: 2012-03-12
Also published as: ATE158752T1; DE69222457T2; JP3046373B2; EP0505209A2; CA2063332A1; KR950007554B1; EP0505209B1; JPH04292944A; NZ242044A; DE69222457D1; EP0505209A3; US5236774A; KR920017810A; AU1286892A

Abstract

A laminate suitable for use as a food-packaging stretchable film comprises an intermediate layer (a) mainly comprising a butene-propylene copolymer, and an upper surface layer (b1) and a lower surface layer (b2) disposed on both sides of said intermediate layer (a), said surface layers mainly comprising a polyolefin resin (B).

Description

P/00/0011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

3 3 7 3 Name of Applicant: MITSUI PETROCHEMICAL INDUSTRIES, ,TD. and SHELL OIL COMPANY Actual Inventor(s): Haruhiko TANAKA and Akito NISHIMURA .00. 6 Address for service in Australia: CARTER SMITH BEADLE Qantus House 2 Railway Parade Camberwell Victoria 3124 Australia Attorney Code CD Invention Title: POLYBUTENE-1 RE SIN LAMINATE The following statement is a full description of this invention, including the best method of forming it known to us: Our Ref: #10252 BMH:WB 03-11mit a14 BACKGROUND OF THE INVENTION This invention relates to a polybutene-1 resin laminate, and more particularly, to a polybutene-1 resin laminate having excellent transparency, stretchability, flexibility, 5 slip property, heat sealability and self-adhesion, which may be stretched without undergoing generation of wrinkles and decrease in its transparency and fastening ability. The *e S polybutene-1 resin laminate of the present invention may be preferably used as a stretchable film for packaging foods.

Fresh and cooked foods are displayed and put on sale as a commodity after a so-called prepackaging to keep the food clean and free from dust deposition and contamination, as well as to maintain the freshness of the food. In the S. prepackaging of the fresh or cooked food, the food is either directly stretch-packaged with a transparent food-packaging stretchable film, or stretch-packaged after placing the food on a plastic tray. The food-packaging stretchable film is stretched upon packaging in order to keep the item to be packaged under strictly sealed conditions. The foodpackaging stretchable film is required to have a good transparency for visible packaging as well as aesthetic, brilliant appearance for improving commercial values of the item packaged within the film. The food-packaging stretchable film is also required to have excellent oxygen and water vapor permeability, and aroma-retaining properties in order to retain the freshness and aroma of the fresh or cooked food, as well as good food-preserving ability at low temperatures. Furthermore, the food-packaging stretchable film is required to have good stretchability and elasticity to endure stretch-packaging with an automatic packaging machine, as well as satisfactory self-adhesion to prevent the film from becoming peeled off when the package is left for a o *O long period after the packaging. As mentioned above, a wide variety of properties are required for the food-packaging stretchable film.

Films comprising a soft polyvinyl chloride (PVC) have been mainly used for the food-packaging stretchable films 15 which are required to have the properties as mentioned above.

The soft vinyl chloride resins, however, are liable to S suffer from such problems as migration of the plasticizing agent contained in the resin into the food, which may result in hygienic problems, and generation of hydrogen chloride gas 0 upon incineration after the disposal. Therefore, use of the vinyl chloride film for the food-packaging stretchable film is nowadays reexamined, and development of a substitute for the polyvinyl chloride film is urgently required As the substitutes for the soft vinyl chloride resin, there have been proposed laminates utilizing the properties of polybutene-l, which has a quite high molecular weight -3among the polyolefins, and has rubbery properties similar to those of the polyvinyl chloride. Such laminates comprise an intermediate layer of polybutene-1 and surface layers of another polyolefin such as polyethylene, ethylene-vinyl acetate copolymer, and the like. For example, food-packaging stretchable films comprising three layers, that is, polyolefin/polybutene-1/polyolefin, ethylene-vinyl acetate o copolymer/polybutene-1/ethylene-vinyl acetate copolymer, and o polyolefin/polybutene-1/polyolefin are disclosed in Japanese 0 Patent Publication No. 62(1987)-27981, Japanese Patent Application KOKAI Nos. 61(1986)-89040 and 1(1989)-166954, respectively.

In the above-described conventional laminates comprising at least three layers, the polybutene-1 used for the 5 intermediate layer is either a homopolymer of butene-1 or a 9 copolymer containing more than 85% by mole of butene-1, resulting in a high crystallinity. Therefore, upon stretch packaging of the laminate film, the polybutene-1 intermediate layer experiences oriented-crystallization causing irregular 26 reflection of the light passing through the layer to result in haze and decrease in transparency of the laminate. Also, the intermediate layer comprising a polybutene-1 homopolymer or a copolymer of high butene-1 content has a poor compatibility with the polyolefin surface layers. As the consequence, upon stretching of the laminate film, the surface layers are separated from the intermediate layer at -4the boundaries therebetween, and the layers are unevenly stretched to cause wrinkles to result in poor appearance.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a polybutene-1 resin laminate which is preferable for a use as a food-packaging stretchable film, which has

A

excellent transparency, stretchability, flexibility, smoothness, heat sealability and self-adhesion, and which may be stretched without undergoing generation of wrinkles and r0 decrease in transparency and fastening ability.

Such an object of the present invention can be attained by a polybutene-1 resin laminate comprising an intermediate layer mainly comprising a butene-l-propylene copolymer i. containing 15 to 40% by mole of propylene, said copolymer having a melt flow rate in the range of from 0.1 to

S.

g/10 min.; and an upper surface layer (bl) and a lower surface layer (b2) disposed on both sides of saia C intermediate layer said surface layers mainly comprising a polyolefin resin (B) to DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The polybutene-l resin laminate (hereinafter simply referred to as "laminate") of the present invention is hereinafter described in detail.

The butene-l-propylene copolymer, which is the main constituent of the intermediate layer in the laminate of the present invention, may have a propylene content of 15 to by mole, preferably 15 to 35% by mole, and most preferably 18 to 30% by mole.

The butene-l-propylene copolymer may have a melt flow rate at 190°C of 0.1 to 30 g/10 min., and preferably 0.3 to 20 g/10 min.

The butene-l-propylene copolymer may preferably have a

O*

J0 ratio of weight average molecular weight [Mw] to number average molecular weight Mw/Mn of 2 to 8 in view of a fair flowability, and to provide good processability and appearance with the resulting film. The rati of the weight average molecular weight [Mw] to the number average molecular 15 weight Mw/Mn may be determined as described below.

Using standard polystyrenes (monodisperse polystyrene, manufactured by Toso whose molecular weight is already known, a calibration curve demonstrating molecular weight, M in relation to elution volume, EV is to.0 depicted by analyzing polystyrenes of various molecular weights by GPC (gel permeation chromatography) under the conditions as shown below.

Measurement Conditions: Equipment: Model 150C manufactured by Water Inc.

Column: TSKGMH-6.6 mm diam. x 606, manufactured by Toso K. K.

-6- Sample volume: 400 gl Temperature: 135°C Flow rate: 1 ml/min.

Polymer sample whose molecular weight is to be measured was charged in a flask with a solvent, odichlorobenzene to prepare a solution comprising 15 mg of the polymer and 20 ml of the solvent.

To the resulting solution was added a stabilizing agent, 2,6-di-t-butylcresol to a concentration of 0.1% by weight.

S* o The resulting solution was heated to 140"C for 1 hour, and agitated for another 1 hour to completely dissolve the polymer and the stabilizing agent in the solution.

The solution was then filtered at 135 to 140°C with a filter of 0.5 pm.

1S The resulting filtrate was analyzed by GPC under the *5 same measurement conditions as the above to measure the EV. The number average molecular weight [Mn]: [Mn] ZMiNi/ZNi, and the weight average molecular weight [Mw]: o0 [Mw] Mi 2 Ni/MiNi were determined from the EV measured by -eferring to the calibration curve depicted in to further calculate the ratio of the weight average molecular weight to the number average molecular weight, Further, the butene-l-propylene copolymer may have a melting point, Tm determined with a differential scanning calorimeter of about 50 to 110°C to impart an excellent heat resistance to the resulting film, and a crystallinity determined by X-ray diffraction of about 5 to 50% to provide a highly stretchable, strong film with a high tear resistance, and preferably about 10 to The melting point, Tm as mentioned above may be measured with a differential scanning calorimeter by elevating the a.

temperature of a sheet, which was pressed to a thickness of 0.1 mm, and has been aged for 10 days, from O'C to 250'C at a 10 temperature elevation rate of 10'C/min., and determining the t* 0 temperr"'-re indicating the maximum endothermic peak.

The crystallinity is determined by subjecting a sheet pressed to a thickness of 1.5 mm which has been aged for days to X-ray diffraction.

i5 The butene-l-propylene copolymer may optionally have Sanother resin blended thereto to a proportion of up to 40% by weight in order to adjust stretchability, flexibility, or tear strength and to improve press processability. Such resins which may be blended with the butene-1-propylene copolymer include a propylene resin containing at least by mole of propylene, an ethylene resin containing at least by mole of ethylene, and a butene-1 resin containing at least 85% by mole of butene-1. Illustrative examples of such a propylene resin include propylene homopolymer, propyleneethylene copolymers, propylene-butene-1 copolymers, and propylene-ethylene-butene-l terpolymers. Illustrative -8examples of such an ethylene resin include high-density polyethylenes, linear low-density polyethylenes, highpressure low-density polyethylenes, ethylene-vinyl acetate copolymers, ethylene-unsaturated fatty acid copolymers, ethylene-a-olefin copolymers, ionomer resins produced by crosslinking an ethylene-methacrylic acid copolymer with a metal ion such as zinc ion and sodium ion. Illustrative examples of the butene-1 resin include butene-i homopolymer S and butene-l-a-olefin copolymers.

*4 £0 The intermediate layer mainly comprising the butene- *4 1-propylene copolymer as described above may typically have a thickness of about 1 to 15 (tm to provide well-balanced strength and stretchability with the resulting laminate, and preferably, a thickness of about 2 to 10 pm in view of good i 5 processability and adaptability to automatic packagjig.

The upper surface layer (bl) and the lower surface layer (b2) disposed on both sides of the intermediate layer may primarily comprise a polyolefin resin The polyolefin resin may typically comprise a 10 homopolymer of an a-olefin containing 2 to 20 carbon atoms, a copolymer of two or more a-olefins containing 2 to 20 carbon atoms, a copolymer of such an a-olefin and a monomer copolymerizable with such an olefin, or a crosslinked polymer of such a homopolymer or copolymer. The polyolefin resin may preferably have a molecular structure similar to that of the polybutene-I to improve the adhesion between -9the intermediate layer and the surface layers (bl) and (b2).

Exemplary monomers which are copolymerizable with such an a-olefin include (tneth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, and vinyl acetate.

Illustrative examples of the polyolefin resin (B) include a polypropylene, a polyethylene, an ethylene-vinyl acetate copolymer, an ethylene-acrylic acid copolymer, an a ethylene-ethyl acrylate copolymer, an ethylene-rnethacrylic *5 *10 acid copolymer, a copolymer of ethylene with another aa.* S olefin, and any one of the above mentioned polymers crosslinked with a metal ion. The copolymer of ethylene with another a-olefin may preferably contain 60 to 95% by mole of the ethylene. The polyolefin resin may comprise either 5 one, or two or more of the above mentioned resins. The a polyolefin resin may most preferably comprise a linear *a *b low-density po',lethylene having a density of up to 0.920 g/cm 3 or an ethylene-vinyl acetate copolymer containing 10 to 20o by weight of vinyl acetate in view f improved 20 flexibility and self-adhesion.

The polyolefin resin may typically have a melt flow rate (MFR) of about 0.1 to 30 g/10 min,, and more preferably a melt flow rate of 0.3 to 20 g/10 min. in view of a good press workability, and to prov:.de a viscosity which is wellbalanced with the viscosity of the buten-1-prolylene copolymer.

Furthermore, the polyolefin resin may preferably have a crystallinity measured by X-ray diffraction of 5 to to provide a well-balanced stiffness and strength with the resulting laminate. The measurement of the crystallinity may be carried out by the same manner as the measurement of the crystallinity of the butene-propylene copolymer as described above.

The polyolefin resin may optionally comprise a surfactant such as a fatty ester of a fatty alcohol, and a 10 polyalkylene ether polyol in order to impart adherability, or anti-fogging or antistatic properties. The polyolefin resin miy contain such a surfactant typically to a content of about 0.5 to 5% by weight.

Each of the upper surface layer (bl) and the lower a IS surface layer which primarily comprise the polyolefin resin may typically have a thickness of about 1 to a m, and preferably, a thickness of 2 to 10 km in view of good processability in film production and adaptability to automatic packaging, and to provide well-balanced strength, 20 stretchability, and processability with the resulting laminate.

The laminate of the present invention may generally have a thickness of 5 to 30 gm, and preferably, a thickness of to 20 pm in view of a good balance between the stretchability and the film strength upon stretch packaging, as well as an improved processability.

-11- The process for producing the laminate of the present invention is not limited to any particular process, and the present laminate may be produced by any of the conventional methods including production of the laminate comprising three layers without any post-treatment such as non-stretching film extrusion, for example co-extrusion blown film process and co-extrusion cast film process, and lamination by extrusioncoating; and production of a stretched film by stretching the *t laminate film either monoaxially or biaxially under heating C(0 subsequent to *he production of the laminate film; among which the non-stretching co-extrusion blown film process being most preferable in view of simple production equipment, and excellent balance between the physical properties in machine and transverse directions of the resulting laminate.

15 The laminate of the present invention is quite suitable for such an application as food-packaging stretchable film or *0 6.

protect film since the present laminate is capable of uniformly stretching while maintaining the shrinkability, and o. since the present laminate has an adequate adherability.

20 The present inver'. on is illustratively described by referring to Examples of the invention and Comparative Examples. The present invention, however, is by no means limited by these Examples.

EXAMPLES

c,.ample 1 -12- A polybutene-1 resin laminate film was produced by coextrusion blown film process. A 4 [m-thick film of a butene- 1-propylene copolymer having propylene content of 20% by mole and melt flow rate at 190'C of 4.0 was used as an intermediate layer. On both sides of the intermediate layer were laminated, as an upper surface layer and a lower surface layer, 4 lim-thick films of an ethylene-vinyl acetate copolymer having vinyl acetate content of 15% by mole and melt flow rate at 190'C of f The resulting laminate film was measured for its haze at o *3 unstretched and 30%-stretched states as described below. The laminate film was also evaluated for its packaging ability by packaging two articles each having a height of 10 cm placed on a tray of foamed polystyrene using an automatic stretch a 15 packaging machine, AW-2600Jr manufactured by Teraoka Seiko and observing the stretchability and appearance of the film after the packaging as described below. The butene-1propylene copolymer used for the intermediate layer was I measured for its ratio of weight average molecular weight to 0o number average molecular weight, Mw/Mn, melting point, and crystallinity as described above. The results are shown in Table 1.

Haze value: Haze value of the film was measured at unstretched and ?2 30%-stretched states to examine the degree of haze caused by -13stretching the film. The film was stretched to such a degree since the degree of stretching may vary in accordance with size and shape of the article packaged, and the film is sometimes partly stretched to a degree as high as 30% or even higher.

Stretchability: The film was observed whether it had been evenly and smoothly stretched upon packaging with the automatic stretch packaging machine.

4.

0* (0 Appearance: The film was checked for the presence of wrinkles and tear after the packaging with the automatic stretch packaging machine.

Examples 2 to 6 .4 The procedure of Example 1 was repeated to produce laminate films having the layer formation and the thickness of the layers as shown in Table 1. The butene-1-propylene copolymer constituting the intermediate layer had the propylene content and the property as shown in Table 1. The o. resulting laminate films were measured for their haze value at unstretched and 30%-stretchd states. The laminate films were also evaluated for their packaging ability by observing the stretchability and the appearance of the film upon packaging. The butene-l-propylene copolymers used for the 2S intermediate layer were measured for their ratio of weight -14average molecular weight to number average molecular weight, Mw/Mn, melting point, and crystallinity as described above.

The results are shown in Table 1.

00 0 6 6* S

S

6 6 6*6 **6 660 0 6.

0666 09 6 0@ 6 6 66 *6 666 S S 60 6 *6 S S S 0 6 5. S S 56** 56 SOC S 6 Jabe I Exampi 2 3 4 6 e Layer formation (th-ickness. o~n)

EVA/BP/EVA

(4/4/4) EVA/BP /EVA 4i4)

EVA/BP/EVA

(3/6/3)

L-LDPE/BP/L-LDPE

(414/14) Ionomer /BP/Ionome (4/4/4)

EVA/BP/EVA

(4/4/4) Butene-1-propylene copolymner used for the intermediate layer Haze value(%) Comonomer MFR, Tm, Crysta- unst- 30%- (content, mol%) (g/10min-) Mw/Mn (*Cj llinity rece .9trtbed propylene 4.0 5.2 102 40 1.2 1.4 (20) propylene 1.0 4.8 75 33 0.9 1.1 (26) propylene 1.0 4.8 75 33 1.0 1.3 (26) propylene 4.0 5.2 102 40 1.2 1.4 (20) r propylene 4.0 5.2 102 40 1.5 1.7 (20) propylene 1.7 4.6 67 28 1.3 1.4 Packaging ability Stretcha- Appeagoodt goode good good good good good good good good good good Notes: EVA: ethylene-vinyl acetate copolymer having a vinyl acetate content of 15% and a melt flow rate (MFR) of BP: butene-1-propylene copolymer.

L-LDPE: linear low density polyethylene having a density of 0.910 g/cm 3 and a melt flow rate of Ionomer: an ionomer resin produced by crosslinking an ethylene-methacrylic acid copolymer with zinc ion, having a melt flow rate of -16- Comparative Examples 1 to 6 The procedure of Example 1 was repeated to produce films having the layer formation and the thickness of the layers as shown in Table 2. The polybutene-1 resin constituting the intermediate layer had t1p comonomer content and the property as shown in Table 2. The resulting films were measured for their haze value at unstretched and 30%-stretched states.

The films were also evaluated for their packaging ability by observing the stretchability and the appearance of the film 49 10 upon packaging. The polybutene-l resins used for the intermediate layer were measured for their ratio of weight average molecular weight to number average molecular weight, Mw/Mn, melting point, and crystallinity as described above.

The results are shown in Table 2.

4e e Comparative Example 7 A linear low-density polyethylene (L-LDPE) monolayer film having a density of 0.910 g/cm 3 and a melt flow rate of 1.0 g/l0min. was evaluated for its haze and packaging ability by repeating the procedure of E:tample 1. The linear low- 2O density polyethylene was also measured for its ratio of weight average molecular weight to number average molecular weight, Mw/Mn, melting point, and crystallinity as described above. The results are shown in Table 2.

O

9 9 9- 9*9 @69 *99 9 9** 9 *9 *4 99 9 9 99 99 99C 99 9 4 99 4 4 9*99*9 9 999 949 4 9 Tablez2 Comparative example ENo.L Polybutene resin used for the intermediate layer Layer formation Comonomer MFR, Tm, (thickness um)Ircontntt, mol%) MILMn ('QL Crystallinity Haze value(%) Packaging ability unst- 30%- Stretcha- Appearetched stretched bilitY rance 1 EVA/homo PB/EVA (4/4/4) 2 EVA/BE/EVA (4/4/4) 3 L-LDPE/BE/L-LDPE (4/4/4) 4 EVA/BP/EVA (4/4/4)

EVA/BP/EVA

(4/4/4) 6 EVA/BP/EVA (4/4/4) homopolymer (0) ethylene ethylene propylene (3) propylene (8) propylene (53) 2.0 1.0 1.0 3.2 2.3 3.2 12 125 55 8.3 112 8.3 112 5.3 122 5.4 118 2.0 1.6 1.8 1.9 1.5 1.2 0.9 20.0 8.3 12.5 9.3 7.4 1.4 2.5 good poora) good poorb) good poorb) good poorb) good poorb) good poorc) 5 4? 5 7 L-LDPE monolayer film partly yielded poord) Notes: homo PB: polybutene-1 homopolymer.

BE: butene-l-ethylene copolymer.

a) partly wrinkled and clouded.

b) partly clouded.

c) surface waviness was observed.

d) wrinkles and surface waviness were observed.

-13- The polybutene-1 resin laminate in accordance with the present invention has excellent transparency, stretchability, flexibility, slip property, heat sealability and selfadhesion. Also: the polybutene-1 resin laminate of the present invention can be stretched without undergoing decrease in its transparency and fastening ability by generation of wrinkles. Therefore, the polybutene-1 resin laminate of the present invention is quite adequate for a food-packaging stret:hable film.

The claims form part of the disclosure of this specification.

a.

a a 0*

I

.0 4

Claims

1. A polybutene-l resin laminate comprising an intermediate layer comprising a butene-l--propylene copolymer containing 15 to 40% by mole of propylene, said copolymer having a melt flow rate in the range of from 0.1 to min.; and an upper surface layer (bl) and a lower surface layer (b2) disposed on both sides of said intermediate layer said surface la yers ~r 4 'cormpdsing a polyolefin resin

2. A polybutene-1 resin laminate according to claim 1, wherein the buitene-l-propyl ene ccpolymer has a ratio of weight average molecular weight 'Mw3 to number a'7-erage molecular wei,:ht rIMnI, Mw/Mn o:f 2 tz) S. 3 A polybut'ene-I resin laminate according to claim 1, wherein the butene-l-propylene copclymer has3 a ml h crysi lnt y of 5 to 0 3 C

4. A polybutenie-l resin laminate according to claim 1, wherein the butene-l-propylene copolymer has ante resin selected from the group consisting of a propylene resin, an ethylene resin and a butene-1 resin blended thereto to a proportion of up to 40% by weight.

6. A polybutene-1 resin laminate according to claim 1, wherein the intermediate layer has a thickness of 1 to *4

7. A polybutene-1 resin laminate according to claim 1, a wherein the polyolefin resin comprises a resin selected from the group consisting of a homopolymer of an a-olefin containing 2 to 20 carbon atom a copolymer of two or more -olefi containing 2 to 20 carbon atoms, a copolymer of two or more a-olefins containing 2 to 20 carbon atoms, a copolymer of the a-olefin and a monomer copolymerizable with the a-olefin, and S a crosslinked polymer of the homopolymer or the copolymer. 9* a .9 S' 8. A polybutene-1 resin laminate according to claim 1, wherein the polyolefin resin comprises a linear low- density polyethylene having a density of up to 0.920 g/cm 3 or 9S an ethylene-vinyl acetate copolymer containing 10 to 20% by weight of vinyl acetate.

9. A polybutene-1 resin laminate according to claim 1, wherein the polyolefin resin has a melt flow rate (MFR) of about 0.1 to 30 g/10 min. -21 A polybutene-1 resin laminate according to claim 1, wherein the polyolefin resin has a crystallinity of to

11. A polybutene-1 resin laminate according to claim 1, wherein each of the upper surface layer (bl) and the lower surface layer (b20) has a thickness of 1 to 15 pm. 12 A polybutene-1 resin laminate according to claim 1 wherein the laminate has a thickness of 5 to 30 pm.

13. A polybutene-1 resin laminate according to any one of the preceding claims wherein the butene-1-propylene copolymer contains from 15 to 35% by mole of propylene.

14. A polybutene-1 resin laminate according to claim 13 wherein the butene-l-propylene copolymer contains from 18 to 30% by mole of propylene.

15. A polybutene-1 resin laminate substantially as hereinbefore described with reference to any one of the non-comparative examples. 0 DATED 25 August 1993 CARTER SMITH BEADLE S* 20 Fellows Institute of Patent Attorneys of Australia Patent Attorneys for the Applicant: MITSUI PETROCHEMICAL INDUSTRIES, LTD. and SHELL OIL COMPANY o2 G' AM:Q:10252,RES 25 803 -22- ABSTRACT OF THE DISCLOSURE A polybutene-l resin laminate adequate for a food- packaging stretchable film is disclosed. The laminate includes an intermediate layer mainly comprising a butene-propylene copolymer, and an upper surface layer (bl) and a lower surface layer (b2) disposed on both sides of said intermediate layer said surface layers mainly comprising a polyolefin resin 1 *0